Top-ranked Serena Williams lost to Ana Ivanovic on Sunday, ending her 25-match winning streak and taking the hottest of title contenders out of the draw.

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Maria Sharapova of Russia follows through on a shot to Dominika Cibulkova of Slovakia during their fourth round match at the Australian Open tennis championship in Melbourne, Australia, Monday, Jan. 20, 2014.

With Maria Sharapova following Serena Williams out of the Australian Open in the second major upset in 24 hours, attention turned quickly to defending champion Victoria Azarenka's bid for a third consecutive title.

Third-seeded Sharapova lost 3-6, 6-4, 6-1 to Dominika Cibulkova in a fourth-round match Monday on Rod Laver Arena, struggling with her serve in the second and third sets and making 45 unforced errors as she tried to claw her way back.

Top-ranked Williams lost to Ana Ivanovic on Sunday, ending her 25-match winning streak and taking the hottest of title contenders out of the draw.

Losing the rivals ranked Nos. 1 and 3 in quick succession didn't distract No. 2 Azarenka on Monday. She had a 6-3, 6-2 win over No. 13-seeded Sloane Stephens in a match that lacked the drama of their contentious semifinal here last year.

"The players who beat those players deserve all the credit because they've been better, so they are dangerous and they are competitive," Azarenka said when asked about how the draw has opened up for her. "For me it's important to just keep focusing on my game. Quarterfinals of a Grand Slam are never easy, no matter who you play."

Men's No. 1 Rafael Nadal advanced in straight sets, but didn't have it all his way in a 7-6 (3), 7-5, 7-6 (3) win over No. 16 Kei Nishikori. He was broken twice in the third set — getting a time violation from chair umpire Evanthia Asderaki in one of them. The 13-time major winner tripped in the first set and tumbled to the court, then had to go to the locker room to change shoes.

Three-time Australian Open finalist Andy Murray ended the run of French lucky loser Stephane Robert, wasting match points in the third set and smashing his racket after the tiebreaker before advancing 6-1, 6-2, 7-6 (6), 6-2. His next opponent will be the winner of Monday's night match between Roger Federer and Jo-Wilfried Tsonga.

Nalal next plays Grigor Dimitrov, who reached the quarterfinals of a Grand Slam for the first time with a 6-3, 3-6, 6-2, 6-4 win over Roberto Bautista Agut.

Until recently, the 22-year-old Bulgarian has been known as "Baby Fed" because of the one-handed backhand he has in common with Roger Federer, and is also widely known as Sharapova's boyfriend. He's making his own way in Melbourne.

Sharapova, only two tournaments into a comeback from a long layoff with an injured right shoulder, started having trouble with her serve in the eighth game, when she was broken at love while serving for the set.

She was broken four consecutive times and fell behind 5-0 in the second set, but reeled off four consecutive games before No. 20-seeded Cibulkova held to level the match. After taking an off-court medical time out after the second set for a hip strain, Sharapova was broken immediately and had seven double-faults in the third set.

"I have to look at the positives and see where I have come from in four or five months. I haven't played a lot of tennis in those six months," said Sharapova, who won the Australian title in 2008 and lost two other finals. "So I certainly would have loved to play a little bit more before playing a Grand Slam, but this is the chance that I was given."

The four-time major winner made it hard for herself in Australia, needing almost 3½ hours to beat Karin Knapp in searing heat the second round — playing 50 minutes between her first and final match points.

Cibulkova completed a full set of Grand Slam quarterfinal appearances with her best run yet in Melbourne. She'll next play first-time Grand Slam quarterfinalist Simona Halep, who beat former No. 1-ranked Jelena Jankovic 6-4, 2-6, 6-0.

Azarenka, who is on an 18-match winning streak at Melbourne Park, will play either No. 5 Agnieszka Radwanska or Spain's Garbine Muguruza.

"I just love playing here, the surroundings, it feels so cozy ... feels like home," she said after her win over Stephens. It was a rematch of their semifinal last year when Azarenka took a medical timeout just when it appeared Stephens was getting momentum, then came back and won the match.

There was very little tension this time, though, after Azarenka broke Stephens' serve to open the second set and then again to take a 5-2 lead.